This invention relates generally to a wavelength monitor and particularly to a wavelength monitor arrangement which senses the wavelength shift of light from a polychromatic light source.
The wavelength distribution of polychromatic light from a light source such as used, for example, in fiber optics angular rate sensors changes scale factor in a relationship proportional to an interferometric sagnac phase shift signal and angular rate. In order to maintain accurate angular rate measurement, an accurate open loop measurement of the shift in wavelength distribution relative to a known calibration set point is required. In a closed loop configuration, a wavelength difference signal is required to drive a deviant polychromatic wavelength distribution back to the calibration set point.
U.S. Pat. No. 4,588,296 issued on May 13, 1986 to Kevin Killian and Richard Schuma and assigned to the assignee of the present invention utilizes a flat reflective diffraction grating arranged with a lens. The present invention uses a concave holographic reflection grating. Further, the aforementioned patent uses a mask configured in an "X" pattern with an alternating set of four transmitting and opaque areas to provide the required transmittance weighting factor at each wavelength. The present invention uses a neutral density filter in the focal plane of the device for this purpose.
The applicant herein is also aware of another device that uses a lens arranged with a flat reflection grating to disperse the polychromatic light across the focal plane of the device. However, instead of utilizing a tilted front surface diffraction grating as does the invention of U.S. Pat. No. 4,588,296, the reflection grating is on the rear surface of a prism element. Also, instead of using a bi-cell detector as does the present invention, a linear array detector is utilized.